Many swimmers who enjoy diving and underwater swimming use snorkels as they make surfacing unnecessary in order to breath as a snorkel is so dimensioned that its upper end can extend above water while the user is partly or wholly submerged.
One difficulty attendant the use of conventional snorkels is that after the swimmer dives, his snorkel becomes filled with water which must be expelled, after surfacing, before he may again fill his lungs with fresh air. In one type of snorkel, the difficulty is lessened by means of a normally closed valve below the mouthpiece which valve opens when the wearer exhales thus to block the entrance of water into the snorkel as long as its upper end is above water.
Another difficulty which snorkel users experience is that the upper end of the snorkel, when fresh air is to be inhaled, may be flooded by a wave. If that happens, the snorkel will be at least partially filled with water with the likelihood that some water will be inhaled so that, at best, the user will again have to exhaust the water from the snorkel before again trying to fill his lungs with fresh air.